It is known that the maximum thrust of jet engines is reached when the ITT (inter stage turbine) limit temperature, or EGT (exhaust gas temperature), and the limit rotational speed N1 of the fan is reached. These two limits ITT and N1 are exceptionally concomitant. In particular, the take off thrust under high outside temperatures is usually limited by the ITT or EGT limit temperatures even though the limit rotational speed N1 of the fan is not yet reached.
Consequently, for an aircraft of a given weight, it is more difficult to take off from an airport located in a hot country than from an airport located in a cold country. The decrease of the maximum thrust of the aircraft in these hot countries leads to an increase of the take off time, thus longer runways and a higher fuel consumption are required.
It is known that at take off and climb, the optimum exit area of the exhaust nozzle of the jet engine to produce the maximum thrust, is different from the optimum nozzle exhaust area required for the cruise flight conditions. The exhaust nozzles of jet engines for business or commercial aircraft are of the fixed type and consequently are designed to ensure the best performance compromise between take off, climb and cruise conditions. When this compromise is more difficult to satisfy, the engine manufacturer equips the engines with convergent/divergent nozzles. The objective of such nozzles is to increase their mass flow coefficient and their thrust coefficient at relatively low engine pressure ratios. These low engine pressure ratios are typical of take off conditions. For flight cruise conditions, the engine pressure ratios are higher and the performance impact of the convergent/divergent is often negative. In order to find a good nozzle operating compromise, the manufacturers work with the percentage of convergence/divergence, so that this percentage will be high enough to improve the thrust during take off, and low enough so that the cruise performance is not too greatly reduced. Yet, this compromise is no more than a stopgap measure, insofar as the maximum thrust of the aircraft is definitively limited by the fixed shape of the nozzle.
It is already known, for aircraft other than business or commercial, and particularly for military aircraft, to use variable area exhaust nozzles. Since the thrust of the aircraft is directly proportional to the area of its nozzle, it is then possible in modulating the exhaust area of the nozzle to choose the optimum thrust adapted to the outside air temperature conditions.
The variable area exhaust nozzles known to date allow large variations of the area of the nozzles, and are comprised of a large number of flaps which prevent their use on business or commercial aircraft, because of their high weight and their complexity. That is why variable area exhaust nozzles have not yet been adapted either on business aircraft or on commercial aircraft.
U.S. Pat. No. 4,577,814 (SNECMA) describes a variable area exhaust nozzle for a military aircraft. This nozzle is extended by a half movable shell supported by a half fixed shell. However, the thrust generated by such a nozzle in the deployed position is misaligned relative to the longitudinal axis of the aircraft. With consideration of a military aircraft, such a misaligned thrust is an advantage which increases the handling of the aircraft. Yet for a business or commercial aircraft, such a misaligned thrust is to be avoided, since the side component of this thrust does not participate to the improvement of the optimum thrust of the engine.
The objective of the present invention is to overcome all these drawbacks, and notably to design a variable area exhaust nozzle for business or commercial aircraft, allowing these aircraft to fly with an optimum thrust in all phases of the flight, i.e. take off, climb, cruise and landing. It is also our objective to increase the value of the maximum thrust, without misaligning it, relative to the axis of the aircraft, and also to design a variable exhaust nozzle easy to manufacture, of low weight, and easy to operate.